A random access channel in a mobile communication system is used for an initial code ranging operation in which a mobile terminal tries to initially access the mobile communication system, a handover code ranging operation in which the mobile terminal moving between base stations enters a target cell, a periodic code ranging operation in which uplink synchronization is periodically adjusted in an orthogonal frequency division multiple random access (OFDMA) mobile wireless access system, and a bandwidth request (BR) code ranging operation of requesting bandwidth allocation.
FIG. 1 is a view illustrating a frame structure used in a mobile communication system, and FIG. 2 is a schematic view illustrating a PN code generator used in the mobile communication system.
Referring to FIG. 1, in the mobile communication system, first three symbols of the uplink frame are used as the random access channels. Among the three symbols, the first two symbols are used as the initial code ranging channel and the handover code ranging channel, and the third symbol is used as the periodic code ranging channel and the bandwidth request ranging channel.
These ranging channels are commonly used by all mobile terminals. In the random access process, transmission is attempted through the ranging channels. In addition, according to ranging purposes, an orthogonal code set is divided and used. The transmission format is equal to a format of a 144-bit orthogonal code symbol generated by the PN code generator shown in FIG. 2.
FIG. 3 illustrates a signaling procedure including the bandwidth request ranging operation for uplink data transmission.
The mobile terminal (hereinafter referred to as a portable subscriber station (PSS)) transmits a bandwidth request code through the random access process. Subsequently, if a base station (BS) receives the bandwidth request ranging code without conflict, the BS allocates such a small uplink region to the PSS that a BR header can be transmitted through a CDMA_Allocation_IE message. When receiving the CDMA_Allocation_BE message, the PSS transmits to the BS a BR header message including a BR amount required for the region allocated by the BS. Next, when receiving the BR header message, the BS drives an uplink scheduler to allocate a bandwidth that is resource dedicated to an uplink data region to the PSS. As a result, the PSS can transmit data to the BS through the allocated bandwidth.